Fig. 38.—Leaves of wood-sorrel.
The bilimbi (Averrhoa bilimbi) is an oriental fruit tree, which may be mentioned here because of its belonging to the same natural order as the humble little wood-sorrel. This tree and some of its movements were known a hundred years ago. The leaves move spontaneously during the day, they move also in response to a touch, being what is termed "sensitive," and finally they subside into a condition of sleep at night. It is said to be a remarkable sight to observe the leaflets of this tree sinking rapidly one after the other, and then slowly rising. At night the leaflets hang down vertically, and are then motionless. By regulating the light in a conservatory, the behaviour of a plant under variations of light was observed. A leaflet was seen to rise in diffused light for twenty-five minutes, and then a blind was removed so that a strong light fell upon it, and within a minute the leaflet began to fall. The descent was accomplished by six descending steps, that is, by falls succeeded by a slight rising, so as to cause a kind of oscillation, each fall being greater than the rise. The plant was again shaded, and a long slow rise commenced, which continued until the sunlight was again admitted. It is unnecessary to enter into all the minute details of this experiment, the object being to show that a rise and fall of the leaflets took place as a consequence of the increase or diminution of direct sunlight. In another chapter we have alluded to the "sensitive" movements of another exotic fruit-tree belonging to the same genus.
The leaves of the common clover (Trifolium repens) are similar in size and form to those of the wood-sorrel, but of a darker green. Their movements nevertheless are quite dissimilar, for the leaflets instead of descending at night, rise and fold over each other so that the under surface is exposed. As evening approaches the two side leaflets twist themselves and move towards each other until their upper surfaces come into contact. At the same time they bend downwards until their midribs form an angle of about forty-five degrees with the upper part of the footstalk. The terminal or middle leaflet rises without twisting, and bends over until it rests upon and forms a kind of roof over the edges of the closed side leaflets. By this means a kind of cone is formed with the apex towards the footstalk (fig. 39). The middle leaflet in this movement passes through an angle of 90° to 140°. The falling of the middle leaflet in this "clover" was observed on the morning of two days. On the first day the leaflet fell between eight o'clock in the morning and three in the afternoon. On the second day it fell between seven o'clock in the morning and one in the afternoon. After this fall the leaflet began to rise again, but slowly, until four o'clock, when the rapid rise for the evening commenced. It is interesting to compare these two little plants with trifoliate leaves, and observe how the same end is attained, by two diverse and opposite means, the one by rising, the other by falling; but both equally decided and remarkable.
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Fig. 39.—Leaflets of clover awake and asleep.
The melilot is another plant with trifoliate leaves, and this exhibits a different class of movements in its nocturnal gyrations. Darwin has thus described the process which he observed. "The three leaflets of each leaf twist through an angle of ninety degrees, so that their blades stand vertically at night, with one lateral edge presented to the zenith. We shall best understand the other and more complicated movements if we imagine ourselves always to hold the leaf with the tip of the terminal leaflet pointed to the north. The leaflets in becoming vertical at night could, of course, twist so that their upper surfaces should face on either side; but the two lateral leaflets always twist so that this surface tends to face the north, but as they move at the same time towards the terminal leaflet, the upper surface of the one faces about N.N.W., and that of the other N.N.E. The terminal leaflet behaves differently, for it twists to either side, the upper surface facing sometimes east, and sometimes west, but rather more commonly west than east. The terminal leaflet also moves in another and more remarkable manner, for whilst its blade is twisting and becoming vertical, the whole leaflet bends to one side, and invariably to the side towards which the upper surface is directed: so that if this surface faces the west, the whole leaflet bends to the west, until it comes into contact with the upper and vertical surface of the western lateral leaflet. Thus the upper surface of the terminal and of one of the two lateral leaflets is well protected." This complicated movement, which it is rather difficult to comprehend at once from a description, is another example of the prodigality of means by which in nature the same end is accomplished.
The common chickweed (Stellaria media) has the leaves in pairs, opposite to each other, and as night approaches these leaves rise upwards with their faces towards each other, the uppermost pair but one closing over the terminal pair, and thus the growing point is protected.
The cultivated nasturtium (Tropaeolum) has a very simple nocturnal motion, which is not observed unless the plants have been well illuminated during the day. These leaves have naturally a tendency to turn to the sun, so that when growing in full light the blade of the leaf is sloped. At night, however, they become vertical by the bending of the footstalk at about an inch below the blade of the leaf. In the morning they resume again their diurnal position. Were it not the fact that these leaves maintain a vertical position during the night, and assume a more horizontal one in the early morning, it might have been thought that the movements in these leaves were only the result of heliotropism, or turning to the sun. A series of experiments has, however, demonstrated that this is not the case, and that the plant has its true nocturnal motion.
Several species of lupins are under cultivation, and some of these exhibit very peculiar movements. In one class the leaflets bend downwards at night, in another, they rise upwards, and, in a third, partly up and partly down. The last is the most curious, for the leaflets are numerous, and spread out like the fingers of an open hand. There may be eight or nine of these leaflets, those nearest the base being the shortest. At night the shorter leaflets, which face the centre of the plant, are depressed, whilst the longest leaflets on the opposite side are elevated, the intermediate ones being slightly twisted. In this way the leaves form during the night a kind of vertical star, the edges of the leaflets being directed towards the zenith, whereas during the day the position of the leaves is horizontal, with the upper surface turned towards the zenith.
A curious circumstance is related of a plant of the yellow lupin (Lupinus luteus), in which different leaves on the same plant went to sleep in a different manner. Two leaves, the leaflets of which at noon stood at about forty-five degrees above the horizon, rose at night to sixty-five or sixty-nine, so that they formed a hollow cone with steep sides. Four leaves on the same plant, which were horizontal at noon, formed vertical stars at night, and three other leaves, which were equally horizontal at noon, had all the leaflets sloping downwards at night. It is difficult to propose a satisfactory solution of these phenomena, for in one plant we have an illustration of all the three methods in which different species of lupin have been observed to pass the night.
The French bean (Phaseolus vulgaris) also exhibits sleep movements, but only under special conditions. With plants growing out of doors no tendency to sleep was observed in July, whereas in August the same plants had most of their leaflets in a condition of repose. In this plant the leaflets sink vertically at night, whilst the footstalk rises a little. Other species of the same genus have been observed, and all of them sleep in a like manner.
We have given as many illustrations as necessary from ordinary and well-known plants, either native, or in common cultivation, and these we would now supplement by a few observations on foreign plants such as are cultivated in the green-house or conservatory. For the facts we shall be indebted here, as on previous occasions, to Mr. Darwin's recent work, which, like his other works, is a perfect cyclopedia of facts and observations.
In Desmodium gyrans, one of the "sensitive plants," so-called, the leaves consist of a large elliptical terminal leaflet, and two very small lateral ones. The large terminal leaflet sinks vertically at night, whilst the footstalk, or petiole, rises. By this rising of the petioles an altered and more compact appearance is given to the plant. The young petioles, near the top of the plant, rise to such an extent as to be nearly parallel to the stem, whilst the older ones rise considerably. In the evening the rising of the petioles is almost completed before the terminal leaflet begins to fall. Whilst the plant is awake—that is, during the day-time, the leaflets are in constant motion; but, after six o'clock in the evening, when the nocturnal descent has commenced, the direction is almost directly and straightly downwards. After the leaflets are completely depressed for the night they move very little or not at all. The little lateral leaflets do not appear to sleep. They were seen in motion, jerking as they usually do, at ten and at eleven o'clock at night. At one o'clock in the morning the leaflets were still jerking rapidly. At half-past three the jerking was not observed. At half-past eight in the morning it had commenced jerking again. "This leaflet, therefore, was moving during the whole night, and the movement was by jerks up to one a.m. (and possibly later), and again at half-past eight a.m."
Similar nocturnal movements, such as the elevation of the petioles, and the vertical depression of the leaves have been observed in several other plants allied to the above, belonging to the same great natural order.
In Acacia Farnesiana the difference between the appearance of a waking and sleeping plant is most remarkable. The leaf is a very compound one (fig. 40) consisting of a petiole with about seven pairs of pinnae, or secondary petioles, each of which is feathered with little leaflets. Towards night the pinnae move forwards, and sink downwards. The leaflets become directed towards the apex of the pinna, and overlap each other, so that the pinnae then look like bits of dangling string (fig. 41). Mere verbal description can give no idea of the contrast between the appearance of this plant as seen by day, and as seen by night.
<!-- image -->Fig. 40.—Leaf of Acacia Farnesiana awake.
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Fig. 41.—Leaf of Acacia Farnesiana in a sleeping condition.
In Coronilla rosea the leaves have nine or ten pairs of opposite leaflets, which during daytime project horizontally. At night these leaflets rise so that the opposite leaflets nearly touch each other, at the same time they bend backwards and towards the stem, sometimes to such an extent that their midribs are parallel to the petiole. This position, both as regards the uprising of the leaflets, and their direction backwards on the petiole, is just the reverse of what usually takes places in the order to which this plant belongs.